Study of the x-ray photoelectron spectrum of tungsten—tungsten oxide as a function of thickness of the surface oxide layer

The photoelectron spectrum of tungsten metal using Al K_α X-rays has been studied as a function of a tungsten oxide layer on the surface. The photoelectron lines arising from the 4f shell of tungsten metal are clearly separated in energy from those coming from WO₃. The ratio of the intensities of these two sets of lines were measured for a series of metal samples which were anodized to a determined level of tungsten oxide. The data were shown to be consistent with a uniform deposition of oxide film. The escape depth, or thickness from which half the photoelectron intensity is derived, was found for a 1450 eV photoelectron to be 8.9 Å and 18.3 Å for W and WO₃, respectively.     

Test Objects with a Rectangular Profile for SEM: 2. Certification of the Groove Width

A procedure for certifying the width of relief grooves with a rectangular profile in silicon is described. It is comprised of two operations. The first operation consists in measuring the thickness of a silicon-oxide film on a single-crystal silicon substrate by ellipsometry. The second operation consists in determining the change in the width of grooves after restoring the native silicon oxide on the surface of the groove side walls. The procedure for measuring the thickness of the native silicon–oxide layer by scanning electron microscopy is described.     

Metal-semiconductor diodes sensitive to silane at room temperature

The electric current in an Au-TiO₂ junction diode under a forward bias is enhanced by introducing silane m the ambient of air at room temperature. The diode is thus capable of selectively detecting silane down to a few ppm. A Pd-TiO₂ junction diode is also sensitive to silane, but a Cu-TiO₂ diode is not. From Auger electron spectroscopic measurement, the formation of silicon oxide on a Au or Pd film exposed to silane was confirmed, but not on a Cu film. The thickness of the silicon oxide laver on the Au film by l h exposure to silane was estimated to be 5 Å.     

The lithiation studies of nanostructured tungsten oxide film prepared via electrochemical precipitation

In this work, nanostructured tungsten oxide was electrodeposited by cyclic voltammetric technique onto a stainless steel surface. The structure and surface morphology of the resulting oxide film were characterized by means of X-ray diffraction, scanning probe microscopy, and scanning electron microscopy. The electrochemical intercalation of lithium into the nanostructured tungsten oxide was studied using cyclic voltammetry, galvanostatic charge–discharge curves, and electrochemical impedance spectroscopy in a liquid electrolyte consisting of 1 M LiClO₄ in propylene carbonate. The as-deposited tungsten oxide indicated the capacity for electrochemical lithium insertion. The specific capacitance of 108.05 F?g^?1 was obtained at the constant discharge density of 0.07 mA?cm^?2.     

n型有序多孔硅基氧化钨室温气敏性能研究

利用电化学腐蚀方法制备了n型有序多孔硅, 并以此为基底用直流磁控溅射法在其表面溅射不同厚度的氧化钨薄膜. 利用X射线和扫描电子显微镜表征了材料的成分和结构, 结果表明, 多孔硅的孔呈柱形有序分布, 溅射10 min的WO₃薄膜是多晶结构, 比较松散地覆盖在整个多孔硅的表面. 分别测试了多孔硅和多孔硅基氧化钨在室温条件下对二氧化氮的气敏性能, 结果表明, 相对于多孔硅, 多孔硅基氧化钨薄膜对二氧化氮的气敏性能显著提高. 对多孔硅基氧化钨复合结构的气敏机理分析认为, 多孔硅和氧化钨薄膜复合形成的异质结对良好的气敏性能起到主要作用, 氧化钨薄膜表面出现了反型层引起了气敏响应时电阻的异常变化. 关键词： 有序多孔硅 氧化钨薄膜 二氧化氮 室温气敏性能     

Thickness dependent magnetic and structural properties of Co films grown on GaAs (100)

The structural, magnetic and transport properties measurements carried out on Co thin films deposited by electron beam evaporation on GaAs substrate as a function of layer thickness ranging from 50 Å to 1000 Å are presented here. Structural measurements show the film to be amorphous in nature at lower thickness which becomes crystalline at higher thickness. Magnetic measurements show an increase in saturation magnetization (M_S) with film thickness. M_S values are found to vary from 521 emu/cm³ to 1180 emu/cm³ for thicknesses ranging from 50 Å to 1000 Å. The coercivity and saturation field value shows a systematic decrease up to 600 Å thickness and increase thereafter. Various microstructural parameters were also calculated using GIXRR technique. A clear grain growth is observed in AFM technique with film thickness and its influence on transport properties was also seen. Different surface morphology and magnetic domain structures were obtained on different thin film samples by AFM and MFM techniques, respectively. XPS measurements reveal formation of CoAs phase at the interface between Co and GaAs. All these results are discussed and interpreted in detail in this communication.     

Persistence of Si(111)7 × 7 surface structure under air exposure

Changes in Si(111) 7 × 7 pattern with air exposure are studied by RHEED. The relation between exposure time and air-oxidized film thickness is obtained using an ellipsometer. As a result, it is found that the 7 × 7 surface after air exposure (760 Torr, 20 h) still exhibits several 7 × 7 spots near normal ones in the RHEED pattern. The air-oxidized film thickness corresponding to this exposure is about 13 Å. The fact that the 7 × 7 spots do not disappear with HF etching of the oxide film indicates that the superstructure remains not at the oxide surface but at the substrate selvedge. The 7 × 7 spots gradually disappear after longer exposure time and no 7 × 7 spots can be observed after 40 h (about 15 Å in oxide thickness). For a sample exposed longer than 40 h, no 7 × 7 spots reappear, even if the oxide is removed by HF. Therefore, it is concluded that the structure of the substrate selvedge changes with exposure time.     

Analysis of semiconductor surfaces with very thin native oxide layers by combined immersion and multiple angle of incidence ellipsometry

In this paper the method of immersion and multiple angle of incidence ellipsometry is used for analyzing silicon and germanium single crystals covered with very thin native oxide layers under normal laboratory conditions. The method is applied at a wavelenght of 632.8 nm. If the values of the absorption index of both semiconductors are known with sufficient accuracy, the values of the refractive index of these semiconductor substrates, the refractive index and the thickness of the effective surface layers (a double layer consisting of the native oxide layer and the interface layer between the semiconductor and this native oxide film) can be determined with relatively high accuracy. These values are compared with those found by other researchers. The thickness and the refractive index of the native oxide layer on the silicon single crystal substrate are also evaluated taking into account the existence of the interface layer.     

Spectroscopic characterization of Ni films on sub-10-nm silica layers: Thermal metamorphosis and chemical bonding

The thermal evolution in the chemical and physical characteristics of the Ni film of thickness 1-50 nm deposited on silica of thickness less than 10 nm was studied. The chemical composition of silica affected the thermal behavior of the Ni overlayer substantially. Nickel deposited on native oxide may diffuse downward into native oxide during annealing and was oxidized. It mainly produced Ni₃O₂ and silicides below 150 °C. Increasing the temperature to 300 °C caused further oxidation of Ni to yield NiO. The sub-10-nm silicon dioxide layer, on the other hand, can inhibit the diffusion of Ni atoms downward when the Ni-deposited sample was annealed. Instead, these atoms aggregated into small particles on the surface at elevated temperatures, causing the substrate to be exposed. The size of the particles produced can be controlled, as it increased almost linearly with the thickness of the Ni film deposited in the low thickness regime. The thinner Ni films yielded smaller, round nanoparticles with better dispersity. The particles formed were strongly adhered to the silicon dioxide surface. The bulk of the particles formed was mainly metallic. Exposing to the air of the Ni particles formed on silicon dioxide mainly produces Ni₂O₃ on the particles.     

Ultra-smooth metal surfaces generated by pressure-induced surface deformation of thin metal films

We present a mechanical pressing technique for generating ultra-smooth surfaces on thin metal films by flattening the bumps, asperities, rough grains and spikes of a freshly vacuum deposited metal film. The method was implemented by varying the applied pressure from 100 MPa to 600 MPa on an e-beam evaporated silver film of thickness 1000 Å deposited on double-polished (100)-oriented silicon surfaces, resulting in a varying degree of film smoothness. The surface morphology of the thin film was studied using atomic force microscopy. Notably, at a pressure of ～600 MPa an initial silver surface with 13-nm RMS roughness was plastically deformed and transformed to an ultra-flat plane with better than 0.1 nm RMS. Our demonstration with the e-beam evaporated silver thin film exhibits the potential for applications in decreasing the scattering-induced losses in optical metamaterials, plasmonic nanodevices and electrical shorts in molecular-scale electronic devices.     

Hydrogen interaction with Al2O3-coated tungsten under plasma irradiation

The effect of amorphous films of Al₂O₃ on deuterium trapping in polycrystalline tungsten under plasma irradiation (150 eV/D) and high fluence (up to 4 × 10²⁴ D/m²) was studied by the methods of nuclear reaction product spectroscopy and thermal desorption spectroscopy. It was determined that the hydrogen permeation blocking properties of aluminium oxide lead to a great increase in deuterium trapped in tungsten. With the complete erosion of the film, this effect continues but with a lower amplitude. Extrapolation of this property to beryllium oxide implies that the presence of beryllium oxide on the surface of the tungsten tiles of the ITER leads to a considerable increase in the tritium inventories accumulated in them.     

The evanescent-wave cavity ring-down spectroscopy technique applied to the investigation of thermally grown oxides on Si(100)

We report about the contribution of thermally grown SiO_x overlayer on the SiO_x/Si interface (with oxidation states Siⁿ⁺, where n = 1, 2, 3, 4) to the optical losses of a resonant spectroscopic cavity. The experiments on Si oxide thin films were performed in evanescent wave for Si samples in contact with a total internal reflection surface of a BK7 prism. The evanescent field can be exploited to investigate properties and processes such as the absorption of thin film or solid/air interface reactions. The results show that the oxide overlayer thickness grows with the thermal exposure time and is limited after more than 7 h of treatment. Transmission electron microscopy has been used for the native oxide thickness measurement and angle-resolved X-ray photoelectron spectroscopy used to determine the thermal oxide thickness. A change of absorption coefficient Δα in the range 100–200 cm^?1 is obtained by evanescent-wave cavity ring-down spectroscopy (EW-CRDS) for thermal silicon oxide overlayer, in agreement with the general trend from literature. The evaluation from the EW-CRDS experiments presents the used setup as a competitive method for measuring the absorption properties of thin overlayer.     

Improved performance of silicon‐nanoparticle film‐coated dye‐sensitized solar cells

Silicon (Si) nanoparticles with average size of 13 nm and orange–red luminescence under UV absorption were synthesized using electrochemical etching of silicon wafers. A film of Si nanoparticles with thickness of 0.75 µm to 2.6 µm was coated on the glass (TiO₂ side) of a dye‐sensitized solar cell (DSSC). The cell exhibited nearly 9% enhancement in power conversion efficiency (η) at film thickness of ～2.4 µm under solar irradiation of 100 mW/cm² (AM 1.5) with improved fill factor and short‐circuit current density. This study revealed for the first time that the Si‐nanoparticle film converting UV into visible light and helping in homogeneous irradiation, can be utilized for improving the efficiency of the DSSCs. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Studies on morphology and molecular arrangement of pentacene on different substrates

Pentacene (C₂₂H₁₄) thin films with different thicknesses were fabricated to study the dynamic growth process and morphology of pentacene on different substrates. A discontinuous monomolecular layer was observed when a pentacene thin film is about 0.5 nm thick on native oxide silicon wafer. The terraced islands and dendritic structure gradually formed with increasing pentacene thin film thickness. The height of each layer is about 1.4 nm which corresponds well with the length of the long axis of the pentacene molecule at 1.45 nm. Experimental results show that the pentacene molecule is perpendicular to the silicon wafer surface with a slight tilted angle. However, the pentacene molecular orientation on a polymer pre-covered indium tin oxide coated substrate could not give any indication on the scale of nanometers. The surface roughness of substrates strongly influences pentacene molecular diffusion and the morphology of pentacene thin films.     

An exploratory study of the reactive Ni-GaAs (1 1 0) interface

Soft X-ray photoemission spectroscopy measurements have been carried out on cleaved n-type GaAs (1 1 0) surfaces covered with Ni overlayers ranging in thickness from 0.05 to 53 Å. The results of these room temperature measurements show that we have band bending effects occurring in conjunction with strong interfacial chemical reactions. Deconvolution of the Ga 3d core line into substrate and metallic components shows dissolution of the substrate at the interface with Ga diffusing into the surface of the metal overlayer for the intermediate coverages (1–15 Å). Observation of the As 3d core level shows out-diffusion of As to the surface over the entire Ni coverage range. Using this deconvolution scheme we are able to follow the band bending of the Schottky barrier formed here up to the 8 Å coverage. The Schottky barrier height is 1.0 ± 0.1 eV for this overlayer thickness.     

Low energy alkali ion scattering investigation of Au nanoclusters grown on silicon oxide surfaces

The atomic and electronic structures of Au nanostructures grown by deposition onto various silicon oxide surfaces were probed with low energy alkali ion scattering. Charge state-resolved time-of-flight spectra of scattered 2 keV ³⁹K⁺ ions were collected from Au deposited onto an untreated Si wafer with a native oxide, a thermally grown oxide surface, and atomically-clean Si(111). It is shown that nanoclusters form on both oxides, but not on the clean Si. A quantitative analysis of the ion scattering spectra indicates that the nanoclusters are initially flat, two-dimensional structures that start to develop a second layer at about 0.5 Å of deposited Au and then form three-dimensional islands. The neutral fraction of scattered 2 keV ³⁹K⁺ ions decreases with deposition indicating changes in the quantum state occupancy with cluster size. The shapes of the clusters differ on the native and thermal oxides, leading to shape-dependent neutralization.     

Features of atomic and electronic structure of oxides on porous silicon surface according to XANES data

Based on synchrotron research of the fine structure main parameters of SiL _{2, 3} X-ray absorption edges (X-ray absorption near edge structure (XANES)) in porous silicon on boron-doped Si(100) wafers, the thickness of the surface oxide layer and the degree of distortions of the silicon-oxygen tetrahedron in this layer were estimated. The thickness of the oxide layer formed on the amorphous layer coating nanocrystals of porous silicon exceeds the thickness of the native oxide on the surface of Si(100) : P and Si(100) : B single-crystal (100) silicon wafers by several times. Distortion of the silicon-oxygen tetrahedron, i.e., the basic unit of silicon oxide, is accompanied by Si-O bond stretching and an increase in the angle between Si-O-Si bonds.     

The role of atomic hydrogen in pre-epitaxial silicon substrate cleaning

The cleaning of silicon (Si) surfaces is a very important issue for the fabrication of novel semiconductor devices on the nanoscale. Established methods for the removal of organic impurities and the native or chemical oxide are often combined with high temperature desorption steps. However, devices with small feature sizes will be unfunctional if, for example, out-diffusion of dopants is not prevented. In this paper we present two possible processes for low-temperature cleaning: an atomic hydrogen source, based on dissociative adsorption of hydrogen at a heated tantalum (Ta) surface and a hydrogen DC plasma source as a part of an UHV cluster tool. The influence of atomic hydrogen on carbon and oxide removal is surveyed and the existing model for native oxide etching with an argon/hydrogen DC plasma is adapted.     

XPS and AES studies of anodic passive films grown on chromium electrodes in sulphuric acid baths

The surface composition and the thickness of anodic passive films formed on chromium electrodes in sulphuric acid baths have been studied by XPS and AES using Ar⁺—ion sputtering. The results are consistent with a 13–16-Å thick oxide layer of composition close to Cr₂O₃. Some evidence is also given concerning hydrated species located in the outermost part of the passive film.     

Determination of the reduction depth of tungsten oxide thin films under the effect of proton irradiation

This work is devoted to experimental determination of the limiting reduction depth of tungsten oxide to metal by the measurement of volumetric variations in a thin film of WO₃ under the effect of proton irradiation with an energy of 1.5 keV. The method of radiation-induced reduction of tungsten from WO₃ can be used for preparation of conducting structures in a dielectric matrix and obtaining an inorganic mask for carrying out different ion-beam processes. It is shown experimentally that the effect of a proton beam with an energy of 1.5 keV provides complete reduction of a tungsten oxide layer up to 138 nm thick. The experimental ratio of the thickness of the reduced layer to the thickness of the starting oxide film was 0.31. It is shown that the limiting reduction depth of tungsten oxide is determined by the path of protons in tungsten.